At present, a discharge type fluorescent lamp and an incandescent bulb used as the illumination device involve problems that a harmful substance such as mercury is contained, and life span is short. However, in recent years, a high luminescence LED emitting light of near ultraviolet/ultraviolet to blue color has been developed in sequence, and the white LED illumination for the practical application of the next generation has been actively studied and developed, by combining the light of ultraviolet to blue color generated from the LED, and a phosphor having an excitation band in a range from ultraviolet to blue color. When the white LED illumination is put to practical use, since efficiency of converting electric energy into light is improved, less heat is generated and it is constituted of a semiconductor device and the phosphor, the white LED has advantages of good life span without burn-out of a filament as is seen in a conventional incandescent bulb, being strong against vibration and repeated on/off operation, and having no harmful substance such as mercury contained therein, thus realizing an ideal illumination device. Further, by utilizing the aforementioned characteristic, the aforementioned white LED is noted as a backlight for a liquid crystal display which replaces a CCFL (cold-cathode tube) other than as illumination.
Here, in order to obtain a white light by combining the aforementioned LED and the phosphor, generally two systems are considered. In one of such systems, white light emission is obtained by combining a blue light emitting LED and a yellow light emitting phosphor (such as YAG:Ce) which emits yellow color by receiving and excited by the blue light emission, and by a principle of mixed state of lights of blue light emission and yellow light emission.
In another system, the white light emission is obtained, by combining a near ultraviolet/ultraviolet light emitting LED, a red (R) light emitting phosphor by receiving and excited by the near ultraviolet/ultraviolet light emission, a green (G) light emitting phosphor, a blue (B) light emitting phosphor, and others, and using a mixed state of the lights of the R, G, B and others emitted from the phosphor. A method of obtaining the white light emission by the lights of the R, G, B and others has wide applications, because an arbitrary light emission color other than the white light can be obtained depending on a combination and a mixing ratio of each phosphor of the R, G, B and others. As the phosphors used in the aforementioned usage, examples are given such as Y2O2S:Eu, La2O2S:Eu, 3.5MgO.0.5MgF2.GeO2:Mn, (La, Mn, Sm)2O2S.Ga2O3:Eu as the red phosphor, ZnS:Cu, Al, SrAl2O4:Eu, BAM:Eu, Mn as the green phosphor, and BAM:Eu, Sr5(PO4)3Cl:Eu, ZnS:Ag, Cl, (Sr, Ca, Ba, Mg)10(PO4)6Cl:Eu as the blue phosphor. Then, by combining the phosphors of the R, G, B, and others with a light emitting part such as the near ultraviolet/ultraviolet light emitting LED, a light source and an illumination device such as an LED emitting white light or the light with desired emission color can be obtained.
However, in the white LED illumination obtained by combining the blue LED and the yellow phosphor (YAG:Ce), the light emission on the longer wavelength side of a visible light becomes insufficient, thus emitting the light with slightly bluish white emission color, and the white light emission with slightly reddish white color like an electric bulb can not be obtained.
In addition, in the white LED illumination obtained by combining the near ultraviolet/ultraviolet LED and the phosphor of R, G, B, and others, the red phosphor out of three color phosphors has a deteriorated excitation efficiency, thereby exhibiting low emission efficiency, compared to other phosphors. Therefore, the mixing ratio of only red phosphor must be increased and the phosphor for improving luminance becomes thereby insufficient, making it impossible to obtain white color with high luminance. Further, a problem involved therein is that the aforementioned red phosphor has a sharp emission spectrum, thereby exhibiting deteriorated color rendering property.
Further, from the viewpoint of improving the emission efficiency of the light emitting element and the phosphor, in the case of the aforementioned YAG:Ce yellow phosphor, the YAG:Ce yellow phosphor is in an excitation range with high efficiency when it is caused to emit light by using blue light emitted from the blue LED, whereby an excellent yellow light emission can be obtained. However, when it is caused to emit light by using the near ultraviolet/ultraviolet light emitted from the near ultraviolet/ultraviolet LED, the YAG:Ce yellow phosphor is out of the excitation range with high efficiency, and therefore an adequate light emission can not be obtained. This means that the excitation range with high efficiency is narrow for the YAG:Ce yellow phosphor.
Then, the problem that the excitation range with high efficiency is narrow for the YAG:Ce yellow phosphor involves a situation such that a wavelength balance of blue color and yellow color is lost, because the emission wavelength of the blue LED is out of an optimal excitation range of the YAG:Ce yellow phosphor by the deviation of an emission wavelength caused by the deviation of the light emitting elements during manufacturing the blue LED even in a case of light emission by using the blue light emitted from the aforementioned blue LED. When the aforementioned situation occurs, the problem involved therein is that a color tone of the white light which is obtained by synthesizing the blue light and the yellow light is changed. Here, in manufacturing the LED, it is difficult to prevent the deviation of the light emitting elements in the present circumstances. Therefore, in order to prevent the change of the color tone, the phosphor having characteristics of a broad and flat excitation band is desired.
Therefore, recently, an oxynitride glass phosphor having an excellent excitation band on the longer wavelength side and capable of obtaining an emission peak with a broad half value width (for example, see patent document 1), and a sialon-based phosphor (for example see patent documents 2 and 3), and a silicon nitride-based phosphor containing nitrogen (for example, see patent documents 4 and 5) are reported. Then, the phosphor containing the nitrogen has a larger ratio of covalent bond than an oxide phosphor, and therefore has an excellent excitation band in the light with the wavelength of 400 nm or more, gathering attention as a white LED phosphor. However, such a phosphor fails in reaching a satisfactory level in the present circumstances.    (Patent document 1) Japanese Patent Laid Open No.2001-214162    (Patent document 2) Japanese Patent Laid Open No.2003-336059    (Patent document 3) Japanese Patent Laid Open No.2003-124527    (Patent document 4) Japanese Patent Laid Open No.2003-515655    (Patent document 5) Japanese Patent Laid Open No.2003-277746